I think it's rather simple for a mod to see a troll post and delete it. A report button would probably raise trolling more than it would remove it. It's very easy to tag a mod, and even easier to simply wait for a mod to delete it. A button that gives a notification really isn't needed.
Besides, we barely get any trolls that even try anymore.
Also, some people have different standards of what should be deleted or not. It would be rather annoying for a mod to look at a reported post only to find that it's not really worthy of deleting, but someone else thought so.
Also, a textbox stating why you reported a post takes the same amount, if not less effort than simply tagging a mod.